Скачать или смотреть How to Create a Serializable and Mutable Singleton in Java

Learn how to implement a Java Singleton that is both `serializable` and `mutable`, allowing you to maintain unique identifiers across your objects.
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This video is based on the question https://stackoverflow.com/q/78156728/ asked by the user 'megidderp' ( https://stackoverflow.com/u/23480292/ ) and on the answer https://stackoverflow.com/a/78158294/ provided by the user 'tgdavies' ( https://stackoverflow.com/u/11002/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem

When developing object-oriented systems in Java, developers often face challenges in ensuring that certain classes are unique across the application. One common design pattern that addresses this issue is the Singleton pattern. Not only does this pattern ensure that only a single instance of the class exists, but it also allows for sharing certain variables throughout the application.

However, many times these classes must also adhere to being serializable so the application state can be saved and restored. Additionally, when it comes to managing identifiers (or IDs), a mutable Singleton that can update its internal state is a necessity.

In this post, we will tackle the challenge of implementing a Singleton class that is both serializable and mutable, utilizing the idCountSingleton class as an example.

The Solution

To create a Serializable and Mutable Singleton in Java, we need to ensure that when the class is deserialized, the singleton instance retains its state instead of creating a new instance. Here’s how to proceed:

Step 1: Define the Serializable Singleton Class

Class Declaration: Start by declaring the class that implements the Serializable interface.

Static Instance: Declare a private static field that will hold the single instance of the class.

ID Management: Include an id variable to manage the unique identifiers.

Here's an example of how this can look in code:

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Step 2: Implementing the readObject Method

To ensure that the static field INSTANCE is set correctly upon deserialization, we can implement the readObject method. This method is called during the deserialization process, allowing us to set the static instance back to the deserialized object:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Serializing and Deserializing the Singleton

Finally, we need to demonstrate how to serialize and deserialize the Singleton instance:

[[See Video to Reveal this Text or Code Snippet]]

Important Points to Remember

Instance Check: The getInstance() method checks for an existing instance to prevent multiple instances from being created.

Incrementing ID: The getAndIncrementId() method allows IDs to be unique and prevents accidental duplication.

Deserialization Handling: By implementing the readObject method, we ensure that the Singleton's state is preserved even after deserialization.

Conclusion

Implementing a serializable and mutable Singleton in Java can solve the issues of maintaining unique identifiers and application state across sessions. By carefully managing instance creation and serialization, we ensure the Singleton behaves as desired, even when saving and restoring the application state.

With this approach in mind, you can confidently implement singletons in your Java applications, ensuring they are flexible and functional. Happy coding!